Lightning devices based on OLED technology are becoming more and more competitive to well known technologies such as fluorescent and inorganic LEDs. An important selling point for the general illumination market is the device scalability within a specific product range: for a given basic characteristic of a lighting device such as a color temperature and brightness, different sizes of such a device are required to illuminate areas of different sizes.
Corresponding to the various sizes of a lighting device, the driver which delivers the necessary current for a specific brightness needs to be adapted as well. Typically, the driver is specifically designed for a given product range or individually for a specific device. However, the usage of such a kind of specialized driver has several disadvantages. For example designing a single driver for a complete range of lighting devices always implies a design for maximum power, i.e., the driver is usually oversized for smaller devices and thus not very efficient. Also, designing a driver individually for every specific lighting device is expensive and increases cost for storage and maintenance, because many different types of drivers have to be managed.
A further drawback is the bulkiness of a single device driver, i.e., it is hard to realize a very thin driver. The thickness of an OLED device, however, is a unique selling point compared to other lighting technologies such as fluorescent and inorganic LEDs. Designing a single driver for an OLED device will destroy this advantage because it is not possible to design a driver of several tens of watts within a thickness of only a few millimeters.
Yet another drawback of the single driver solution is the fact, that high currents need to be distributed over large areas. For example, an OLED device with an efficiency of 50 μm per watt and a required brightness of 1,500 μm requires a total power of P=1,500 lm/50 lm/W=30 W. Assuming an OLED characteristic with a forward voltage of 3 V, the total current flowing is I=1 A/m2. A square resistance of Rsq=1Ω would result in a voltage drop of about Rsq·I/2=0.5 V. The result of such a voltage drop would be a significant brightness drop, which is an unacceptable effect in commercial applications.
In order to improve the situation, it is beneficial to have more than one injection point for the current to avoid unwanted voltage drops.
For example US 2004/0105264 A1 discloses a method and apparatus comprising a multiple light source illuminating device using LEDs.
WO 96/19093 discloses a current consumption control system for controlling the current draw of an electric power consuming device, e.g., a modular strip lighting unit for a lighting system including at least one said unit, in which the strip lighting unit includes at least one light element operatively coupled to the lighting unit.